i personally think that API makes the most reliable of ALL test kits out there...............im assuming ur asking cuz u have inverts? or loaches? well, u could get a TDS meter which would help too. they have a few decent ones on ebay for like $15-$20.............
API is the most reliable to most ppl, unless ur making a statement only about the copper, but i've never used that but i have used the rest like ammonia, nitrates, nitrites, phosphate, pH, high range pH, etc. and no probs. for me evar